DESCRIPTION

utimensat()
and
futimens()
update the timestamps of a file with nanosecond precision.
This contrasts with the historical
utime(2)
and
utimes(2),
which permit only second and microsecond precision, respectively,
when setting file timestamps.

With
utimensat()
the file is specified via the pathname given in
pathname.
With
futimens()
the file whose timestamps are to be updated is specified via
an open file descriptor,
fd.

For both calls, the new file timestamps are specified in the array
times:
times[0]
specifies the new "last access time" (atime);
times[1]
specifies the new "last modification time" (mtime).
Each of the elements of
times
specifies a time in seconds and nanoseconds
since the Epoch (00:00:00, 1 Jan 1970, UTC),
in a structure of the following form:

Updated file timestamps are set to the greatest value
supported by the file system that is not greater than the specified time.

If the
tv_nsec
field of one of the
timespec
structures has the special value
UTIME_NOW,
then the corresponding file timestamp is set to the current time.
If the
tv_nsec
field of one of the
timespec
structures has the special value
UTIME_OMIT,
then the corresponding file timestamp is left unchanged.
In both of these cases, the value of the corresponding
tv_sec
field is ignored.

If
times
is NULL, then both timestamps are set to the current time.

Permissions requirements

To set both file timestamps to the current time (i.e.,
times
is NULL, or both
tv_nsec
fields specify
UTIME_NOW),
either:

1.

the caller must have write access to the file;

2.

the caller's effective user ID must match the owner of the file; or

3.

the caller must have appropriate privileges.

To make any change other than setting both timestamps to the
current time (i.e.,
times
is not NULL, and both
tv_nsec
fields are not
UTIME_NOW
and both
tv_nsec
fields are not
UTIME_OMIT),
either condition 2 or 3 above must apply.

If both
tv_nsec
fields are specified as
UTIME_OMIT,
then no file ownership or permission checks are performed,
and the file timestamps are not modified,
but other error conditions may still be detected.

utimensat() specifics

If
pathname
is relative, then by default it is interpreted relative to the
directory referred to by the open file descriptor,
dirfd
(rather than relative to the current working directory of
the calling process, as is done by
utimes(2)
for a relative pathname).
See
openat(2)
for an explanation of why this can be useful.

If
pathname
is relative and
dirfd
is the special value
AT_FDCWD,
then
pathname
is interpreted relative to the current working
directory of the calling process (like
utimes(2)).

If
pathname
is absolute, then
dirfd
is ignored.

The
flags
field is a bit mask that may be 0, or include the following constant,
defined in
<fcntl.h>:

AT_SYMLINK_NOFOLLOW

If
pathname
specifies a symbolic link, then update the timestamps of the link,
rather than the file to which it refers.

RETURN VALUE

On success,
utimensat()
and
futimens()
return 0.
On error, -1 is returned and
errno
is set to indicate the error.

ERRORS

EACCES

times
is NULL,
or both
tv_nsec
values are
UTIME_NOW,
and:

*

the effective user ID of the caller does not match
the owner of the file,
the caller does not have write access to the file,
and the caller is not privileged
(Linux: does not have either the
CAP_FOWNER
or the
CAP_DAC_OVERRIDE
capability); or,

(utimensat())
pathname
is a relative pathname, but
dirfd
is neither
AT_FDCWD
nor a valid file descriptor.

EFAULT

times
pointed to an invalid address; or,
dirfd
was
AT_FDCWD,
and
pathname
is NULL or an invalid address.

EINVAL

Invalid value in
flags.

EINVAL

Invalid value in one of the
tv_nsec
fields (value outside range 0 to 999,999,999, and not
UTIME_NOW
or
UTIME_OMIT);
or an invalid value in one of the
tv_sec
fields.

EINVAL

pathname
is NULL,
dirfd
is not
AT_FDCWD,
and
flags
contains
AT_SYMLINK_NOFOLLOW.

ELOOP

(utimensat())
Too many symbolic links were encountered in resolving
pathname.

ENAMETOOLONG

(utimensat())
pathname
is too long.

ENOENT

(utimensat())
A component of
pathname
does not refer to an existing directory or file,
or
pathname
is an empty string.

ENOTDIR

(utimensat())
pathname
is a relative pathname, but
dirfd
is neither
AT_FDCWD
nor a file descriptor referring to a directory;
or, one of the prefix components of
pathname
is not a directory.

EPERM

The caller attempted to change one or both timestamps to a value
other than the current time,
or to change one of the timestamps to the current time while
leaving the other timestamp unchanged,
(i.e.,
times
is not NULL, both
tv_nsec
fields are not
UTIME_NOW,
and both
tv_nsec
fields are not
UTIME_OMIT)
and:

*

the caller's effective user ID does not match the owner of file,
and the caller is not privileged
(Linux: does not have the
CAP_FOWNER
capability); or,

CONFORMING TO

NOTES

On Linux, timestamps cannot be changed for a file marked immutable,
and the only change permitted for files marked append-only is to
set the timestamps to the current time.
(This is consistent with the historical behavior of
utime(2)
and
utimes(2)
on Linux.)

On Linux,
futimens()
is a library function implemented on top of the
utimensat()
system call.
To support this, the Linux
utimensat()
system call implements a non-standard feature: if
pathname
is NULL, then the call modifies the timestamps of
the file referred to by the file descriptor
dirfd
(which may refer to any type of file).
Using this feature, the call
futimens(fd, times)
is implemented as:

utimensat(fd, NULL, times, 0);

BUGS

Several bugs afflict
utimensat()
and
futimens()
on kernels before 2.6.26.
These bugs are either non-conformances with the POSIX.1 draft specification
or inconsistencies with historical Linux behavior.

*

POSIX.1 specifies that if one of the
tv_nsec
fields has the value
UTIME_NOW
or
UTIME_OMIT,
then the value of the corresponding
tv_sec
field should be ignored.
Instead, the value of the
tv_sec
field is required to be 0 (or the error
EINVAL
results).

*

Various bugs mean that for the purposes of permission checking,
the case where both
tv_nsec
fields are set to
UTIME_NOW
isn't always treated the same as specifying
times
as NULL,
and the case where one
tv_nsec
value is
UTIME_NOW
and the other is
UTIME_OMIT
isn't treated the same as specifying
times
as a pointer to an array of structures containing arbitrary time values.
As a result, in some cases:
a) file timestamps can be updated by a process that shouldn't have
permission to perform updates;
b) file timestamps can't be updated by a process that should have
permission to perform updates; and
c) the wrong
errno
value is returned in case of an error.

*

POSIX.1 says that a process that has write access to the file
can make a call with
times
as NULL, or with
times
pointing to an array of structures in which both
tv_nsec
fields are
UTIME_NOW,
in order to update both timestamps to the current time.
However,
futimens()
instead checks whether the
access mode of the file descriptor allows writing.